1. Technical Field
The present invention relates generally to ignition coils for developing a spark firing voltage that is applied to one or more spark plugs of an internal combustion engine and more particularly to an ignition coil with a polyimide case or secondary winding spool.
2. Description of the Related Art
Ignition coils utilize primary and secondary windings and a magnetic circuit. The magnetic circuit may include a core formed of steel laminations, as disclosed in U.S. Pat. No. 5,870,012 to Sakamaki et al. Sakamaki et al. disclose an ignition coil having a relatively slender configuration adapted for mounting directly above a spark plugxe2x80x94commonly referred to as a xe2x80x9cpencilxe2x80x9d coil. The ignition coil of Sakamaki et al. has a core composed of laminations of iron plates nearly circular in radial cross-section. Sakamaki et al. further disclose a bobbin disposed radially outwardly of the core having a primary coil wound thereon, another bobbin disposed radially outwardly of the primary coil having a secondary coil wound thereon, and a case disposed outwardly of the secondary coil. An outer or side core is outwardly of the windings. An ongoing problem, however relates to space. The bobbins and case occupy valuable space. This has the result of a larger ignition coil. If there are restrictions or limitations on the overall outside diameter of the ignition coil, the space occupied by the bobbins and case displace, in-effect, spaced or volume that could otherwise be occupied by energy storage materials. Thus, for example, the central core volume may be reduced accordingly, thereby reducing ignition coil performance, or, perhaps, requiring that expensive magnets be included in the magnetic circuit to meet performance requirements. In some instances, the combination of a very limited overall outside diameter and a predetermined energy delivery level simply cannot be met using conventional approaches.
One approach taken in the art involves an ignition coil configuration wherein the primary winding is outwardly of the secondary winding, and used in connection with a multi-piece case to obtain a reduced thickness wall in the area of the HV transformer. However, energy density concessions are made with this approach, as described in greater detail below.
Thus, an ignition coil configuration such as disclosed in Sakamaki et al. (i.e., a design where the secondary winding is outwardly of the primary winding, and a shield outwardly of the case) provides a higher energy delivery capability than an ignition coil where the primary winding is wound external to the secondary winding. This is due principally because the primary winding can be wound directly on the central core, thereby eliminating the need for a primary winding spool, and thus allowing for an increased size central core. However, the foregoing arrangement imposes a high dielectric stress on the case located between the secondary winding and the side core or shield. That is, the electric field (E-fields) intensity is too high for prolonged and repeated exposure to the case. This condition leads to material failure (e.g., and allowing corona discharge), which in turn results in failure of the ignition coil as a whole if allowed to continue unabated.
There is therefore a need for an improved ignition coil assembly that minimizes or eliminates one or more of the problems set forth above.
One advantage of an ignition coil assembly according to the invention is that it provides a smaller coil design with respect to overall outside diameter. Another advantage of the present invention is that it provides a lower cost ignition coil by allowing a larger central core of either a reduced cost steel (e.g., M-27 instead of M-6, as is conventionally used for a central core) or, alternatively, by allowing one or more permanent magnets to be removed (i.e., maintain the same performance by providing a larger core, thereby allowing removal of the one or more permanent magnets). These and other advantages, features and objects are realized if either the case alone comprises polyimide material, or the secondary spool alone comprises polyimide material. If both the case and secondary winding spool comprise polyimide material, then the advantages and benefits are increased along with the additional advantage of having an increased temperature capability ignition coil.
An ignition coil assembly according to the invention includes a central core, a primary winding, a secondary winding, and a case. The central core is generally cylindrical and is formed along a main axis. The primary winding is disposed about the central core and is connected to a power source. The secondary winding is wound on a spool that is configured to be connected to a spark plug. The secondary winding is located radially outwardly of the primary winding. The case is located radially outwardly of the secondary winding and comprises a tube formed of polyimide material.
In a preferred embodiment, the spool comprises a second tube formed of polyimide material.